The UNESCO website has a list of some important dates in the development of radio. The last date on the list is 24th March 1980.
24 March 1980 : Anniversary of the murder of Archbishop of San Salvador, Oscar Arnulfo Romero. He was a great communicator and demonstrated radio's potential to promote and defend human rights.Archbisop Romero's Sunday homilies, in which he spoke of and offered an evaluation of the current events in his country, were broadcast on the diocesan radio station. They were listened to assiduously by his people.
The reasons for the celebration of a World Radio Day are outlined on the UNESCO website:
Radio is the mass media reaching the widest audience in the world. It is also recognized as a powerful communication tool and a low cost medium. Radio is specifically suited to reach remote communities and vulnerable people: the illiterate, the disabled, women, youth and the poor, while offering a platform to intervene in the public debate, irrespective of people’s educational level. Furthermore, radio has a strong and specific role in emergency communication and disaster relief.Vatican Radio's coverage for the day is as follows: Vatican Radio's inauguration: a sound picture , Christian Radio, the Virgin and the bomb and Pius XII : a microphone to enable human hearts to beat as one.
My own view is that radio is a medium that demands a higher level of engagement by the person who listens than does television. It is less passive, and more active. It is also a medium that asks for sustained attention, rather than the short seconds-only attention span expected by much visual media today. For that reason, the pictures are better on the radio - and they go round corners, too, so you can still be listening while you are in another room doing the chores ...